Although plastics structural components generally possess good mechanical and in some cases also good optical properties (such as for example transparency in the case of polycarbonate), most industrial plastics are electrical insulators.
The combination of mechanical properties such as stability, optical properties such as transparency and electrical properties such as electrical conductivity in the case of transparent plastics can provide enormous advantages for a wide range of applications. Foremost among these is the transparency of components, which should be as high as possible in many areas of application, for example for windows for the automobile construction industry or for use in buildings, or for inspection windows in instruments that are intended to be coupled with extended electrical applications (electrical heating, screening electromagnetic radiation, dissipation of surface charge). At the same time, in most cases the mechanical stability of the basic material, and also the freedom of design with regard to the ultimate shape, should be as high as possible. Since the line width can be chosen to be very small, use in the field of solar cell technology (photovoltaic units) as highly conductive electrical conductors is also a real possibility. The advantages in this case are the result of the low surface covering due to the presence of the electrical conductor on the face turned towards the source of light.
By using inks filled with metal particles in the nanometre range, thin electrical conductive strips with virtually any geometry at all can be printed for example on plastic films, for example using ink-jet technology. In this case it is especially desirable that the line width of such conductor strips should be about 20 μm, or even less. At around this limit structures are generally very difficult to detect with the human eye and no troublesome optical effect is produced in transparent components.
A further possible method of producing conductor strips having the fineness described above, i.e. which are visually difficult to detect or cannot be detected at all on surfaces (structures with a line width <20 μm), in particular on polymeric substrates, is to provide the substrate in a pre-treatment step with the necessary structuring in order subsequently to fill the resultant structuring with the conducting material.
However, the following further requirements must also be satisfied if possible, though not all of these can be met with the hitherto known inks.
The electrical conductor strips must be heat-resistant (i.e. stable up to 400° C. for short periods) and also mechanically flexible, the metal particle inks used to produce them should permit the production of strips with a line thickness of 100 μm and less (down to 20 μm), and they should be more convenient and easier to process than known conductive pastes. This means that the paste must have a significantly lower viscosity if ink-jet methods are to be used, and must also have a suitable wetting and spreading capacity and contain smaller particles than conventional conductive pastes. In particular the wetting behaviour and spreading capacity must be taken into account in connection for the aforementioned possibility of filling the depressions of pre-structured surfaces. For such a special use the ink should have, in addition to the properties specified above, also a low contact angle (<45° C.) on the selected substrate and/or as large a surface tension as possible (>10 mN/m).
In particular a specific electrical conductivity of the printed, dried and thermally treated ink of significantly better than 102 Ω−1·cm−1 should also be achieved. The thermal treatment should in particular be carried out at a maximum temperature of 140° C., i.e. the softening point of for example polycarbonate. In addition the conductor strips that are formed should be as mechanically flexible as possible so that they retain the conductivity even when the material expands. In particular the conductor strips should also exhibit a particularly good adhesion to commonly used substrates, especially to polycarbonate.
A very specific requirement placed on such an ink is that the particle size of the metal particles should be significantly less than 20 μm and that the ink should have a low viscosity (less than 150 mPa·s). It also appears to be advantageous if the particles employed in the formulation form close packing arrangements after the printing procedure, which already at low concentrations and low processing temperatures lead to the desired conductivity of the printed structure.
A further special alternative object consists in finding, for the material polycarbonate, a suitable way of producing as simply as possible reflecting surfaces on three-dimensional structures of the polycarbonate. A conventional process used hitherto for this purpose is for example sputtering (physical vapour deposition, PVD) with aluminium or other metals that can be vaporised or atomised under these conditions, though this has disadvantages when covering three-dimensional structures with a reflective coating (non-uniform covering of the matrix to be sputtered), and also requires the use of relatively complicated apparatus (working under reduced pressure, use of vacuum techniques and pressure locks). Furthermore, the PVD and atomisation processes have disadvantages as regards the strength of adhesion of the sputtered layer on the substrate. Thus, the metal layers that are produced cannot be touched directly without first applying a protective lacquer, since this would destroy the layers.
R. W. Vest (Metallo-organic materials for Improved Thick Film Reliability, Nov. 1, 1980, Final Report, Contract No. N00163-79-C-0352, National Avionic Center) describes a printable formulation for conductor strips, but the temperature required here to produce a conductivity is 250° C., i.e. well above the possible application temperature for many industrial plastics.
U.S. Pat. No. 5,882,722 and U.S. Pat. No. 6,036,889 describe a conductive formulation that contains metallic particles, a precursor and an organic solvent and that forms conductive structures only at a temperature of 200° C. and above. Also the viscosity of the formulation in this case is so high that basically this formulation cannot be processed in an ink-jet printer.
Formulations based on readily decomposable silver compounds as low-viscosity solutions with a low sintering temperature are the subject of the document WO 03/032084 (A2). Low decomposition temperatures are also disclosed here, but the specific conductivities of the structure obtained are not mentioned. The lowest temperature at which a conductive coating can be achieved using a silver formulation is cited as 185° C.
The specifications WO-2003/038002 A2 and US-A-2005/0078158 describe formulations with silver nanoparticles, which are stabilised inter alia with sodium cellulose methylcarboxylic acid. Although these specifications describe the necessity for a post-treatment, for example by heat or flocculating agents, they do not disclose any processing temperatures nor the conductivity of the microstructures obtained from the formulation. In addition the exact distribution of the used and obtained nanoparticles is not disclosed, although the size range should be less than 100 nm. The content of the disclosed formulations is not more than 1.2 wt. %. It is however indicated that proportions of 60-74 wt. % might be possible. It is also stated that these are not suitable for ink-jet printing due to the sharp increase in viscosity of the resultant formulation. An upper limit of the content in connection with which a use of the ink would still be possible is not disclosed.
In WO 2006/072959 A2 a method is disclosed for obtaining metal nanoparticles which could be used for example for ink-jet printing. In this connection nanoparticles of sizes less than 20 nm and of unknown size distribution are obtained. A bimodal distribution is not disclosed. The usable contents of the resulting formations are in the range from 0.5 to 80 wt. %. Furthermore, it is disclosed that in the production process a preliminary reduction of silver acetate by means of water-soluble polymers is necessary in order to prevent, inter alia, the agglomeration of the obtained nanoparticles. It is therefore obvious that also in the resulting formulation the polymers employed for the preliminary reduction still interact with the particles or are bound to the latter, in order that the effect described above can be retained.
Overall a more complicated process up to the formulation of an ink is disclosed, including a preliminary reduction, main reduction, concentration by evaporation and final formulation, which means that a large-scale, cost-efficient applicability cannot be assumed.
Another route for forming colloidal metal nanoparticles is disclosed in US-2004/0147618. Monomodal distributions of metallic nanoparticles in sizes between 2 and 10 nm are obtained, by dissolving a metal salt together with a water-soluble polymer in a solvent and treatment with radiation under a protective gas (e.g. nitrogen or argon). The use of the obtained dispersion as an ink is mentioned. However, no formulation for such an ink is disclosed. In particular, the amounts of metal particles that can be used in practice in an ink formulation are not given. Also, in this case too the production of the ink by working under a protective gas is very complicated for a large-scale process.
In order to achieve a low sintering temperature, a mixture of silver and gold nanoparticles is employed in WO 2005/0136638. Good conductivity is mentioned here starting from a sintering temperature of 200° C.
The document EP 1 493 780 A1 describes a silver paste that is a very good conductor after thermal treatment at 150° C., though this formulation is too highly viscous for it to be able to be used in ink-jet printing.
The Cabot company offers the product “Cabot Inkjet Silver Conductor AG-IJ-G-100-S1”, which is a silver conductive ink that can be applied using ink jet technology. Tests on adhesion to plastics such as polycarbonate are not mentioned in the existing documents.
The HARIMA company offers the product line “NP Series Nano-Paste”, which is a silver conductive ink based on nanoparticles and having a low viscosity. However, HARIMA specifies sintering temperatures of 210° C.-230° C. The processing temperature means that the paste is unsuitable for the printing of polymers.